Polyarylene sulfide (PAS) resins, especially polyphenylene sulfide (PPS) resins, are well known as engineering resins having high rigidity, excellent mechanical strength, heat-resistance, etc. and useful as materials for various electronic and electric appliances and others for which high rigidity is required. These resins are conventionally prepared by reacting a dihaloaromatic compound such as p-dichlorobenzene with a sodium-sulfur compound such as sodium hydrosulfide in an aprotic organic solvent such as N-methyl-2-pyrrolidone (hereinafter occasionally referred to as NMP). In this process, however, the formed sodium chloride is insoluble in the reaction solvent such as NMP and, therefore, it is inevitably incorporated in the produced resin, which can hardly be removed by washing.
When lithium compound is used instead of sodium compound, the lithium chloride formed as a by-product is soluble in many aprotic organic solvents including NMP and, therefore, the lithium content in the produced resin is easily reduced. Thus employment of a lithium compound is now spotlighted.
In U.S. Pat. No. 4,451,343 it disclosed that PAS resins including PPS resin can be produced batch-wise or continuously by the reaction of lithium N-methylaminobutyrate (hereinafter occasionally referred to as LMAB), which is formed by the reaction of lithium hydroxide and NMP, a dihaloaromatic compound such as p-dichlorobenzene and hydrogen sulfide. This process is advantageous in that the amount of an alkali metal taken into the produced polymer is far smaller in comparison with the process in which solvent-insoluble sodium chloride is formed, since the polymer such as the PPS resin is produced by using LMAB, which is a lithium compound, as a starting material instead of sodium compound and thus solvent-soluble lithium chloride is formed.
However, lithium is far more costly in comparison with sodium. Therefore, in order to save the production cost, it is imperative that the by-product lithium compound is recovered and reused.
In the above-outlined known process, the LMAB (lithium N-methylaminobutyrate) is used in the form of a reaction mixture obtained by the reaction of lithium hydroxide (LiOH.H.sub.2 O) and NMP.
Lithium hydroxide is sparingly soluble in NMP and, therefore, the mixture is a slurry, which is not convenient in transportation, agitation, etc. in comparison with a solution.
Also, in said process, lithium hydroxide is used as a starting material and lithium chloride is formed in the production of the polymer. When the formed lithium chloride is recovered and reused, it must be converted to lithium hydroxide by any means. It is described in said U.S. patent specification that the lithium chloride aqueous solution recovered from the reaction mixture after the preparation of the polymer and the produced lithium chloride is converted to lithium hydroxide by the reaction with sodium hydrogen carbonate or by means of electrolysis.
However, conversion of lithium chloride to lithium hydroxide for reuse, including the above-mentioned processes, is not desirable from the viewpoint of production cost.
That is, the procedure to form lithium N-methylaminobutyrate from lithium hydroxide and N-methyl-2-pyrrolidone is not satisfactory as a method for preparing high purity lithium N-methylaminobutyrate and as a step of a process for preparation of PAS resins including PPS resins, since the purity of the lithium N-methylaminobutyrate is low (91.5% or so) and thus removal of insoluble impurity lithium compounds from the produced polymer by washing is difficult and it is very disadvantageous to convert the recovered lithium chloride to lithium hydroxide.
Therefore, it would be most desirable to develop a process for preparing high purity N-methylaminobutyrate from N-methyl-2-pyrrolidone and further to prepare PAS resins including PPS resins by forming the starting material lithium N-methylaminobutyrate not from lithium hydroxide but from lithium chloride, which is recovered from the polymer product mixture.
Under the circumstances, in order to solve the above problems, we conducted an intensive study in search of a process to efficiently prepare high purity lithium N-methylaminobutyrate, which can be advantageously used as a starting material for production of PAS resins including PPS resins, from N-methyl-2-pyrrolidone and lithium chloride.
The present invention was made under the above-described circumstances.
The object of the invention is to provide a process for efficiently preparing high purity lithium N-methylaminobutyrate from N-methyl-2-pyrrolidone using lithium chloride instead of lithium hydroxide, and thus enabling easy recycling of the by-product lithium chloride and purification of the polymer product, which were problems in the known process for production of PAS resins including PPS resins by the reaction of lithium N-methylaminobutyrate, a dihaloaromatic compound and hydrogen sulfide.
Thus we have found that high quality N-methylaminobutyrate-pyrrolidone, which satisfies to achieve the above mentioned object, can be prepared by reacting N-methyl-2-pyrrolidone with an alkali metal hydroxide other than lithium hydroxide in a suitable solvent (usually aqueous solution) to form an alkali metal (other than lithium) salt of N-methylaminobutyrate, removing water from the reaction product mixture and adding lithium chloride thereto to convert the alkali salt to the lithium N-methylaminobutyrate, separating the formed by-product alkali metal (other than lithium) chloride by deposition, etc.